Porosity in mudstones and its effectiveness for sealing carbon capture and storage sites

Abstract

Mudstones represent top-seals for many carbon capture sites as they tend to have the correct petro-physical properties, including suitable porosity and pore-size distribution. The pore network of mudstones is thus pivotal for many carbon capture and storage (CCS) projects. The key to understanding the effectiveness of top-seals is an appreciation of the controls on the pore network. For this reason, schemes to classify pore body size, pore type and pore throat size are presented. Pore types include primary and secondary interparticle and intraparticle pores and pores associated with organic matter and fractures. The most relevant mudstone pore body sizes for CCS top seals are likely to be between,62 µm and 1 nm. Pore throat sizes are classified as nano-(,10 nm), transition-(10 nm–0.1 µm), meso-(0.1–0.625 µm), and macro-pore throats (.0.625 µm). Petro-physical, geochemical, and geomechanical properties control porosity and the CO2 sealing integrity of mud-stones; these properties are, in turn, controlled by the rate and extent of compaction, mineral diagenesis and overpressure. The success of a CCS top-seal relies on pore throats in intact top-seal being sufficiently small, and fracture pressure (typically minimum horizontal stress, σhmin) not being exceeded by CO2 pressure. CO2 sorption, especially by smectite in top-seals, may improve the nanoscale sealing efficiency of clay minerals. The systematic workflow presented here will help facilitate the new drive to understand mudstone properties, as they are essential for establishing safe and durable CO2 containment.